1. Field of Invention
This invention is concerned with predistortion circuits used for linearizing power amplifiers, and more particularly, with such circuits which operate at microwave frequencies.
2. Description of Prior Art
Microwave power amplifiers of the traveling wave tube type or solid state type used in ground station transmitters and communication satellites should, ideally, be highly efficient and provide linear amplification. The performance of these amplifiers is undesirably limited by amplitude and phase nonlinearities. In order to reduce amplitude and phase distortions of the signals generated in the microwave power amplifiers and obtain higher carrier to intermodulation distortion products (C/I) ratios, amplifiers are conventionally operated well below saturation with consequent loss of efficiency.
In traveling wave tube amplifiers (TWTA) or solid state power amplifiers (SSPA), for example, there are two sources of nonlinearities: (a) amplitude nonlinearity and (b) phase nonlinearity. The intermodulation distortion (IMD) generated by phase nonlinearity is orthogonal to IMD generated by amplitude nonlinearity.
Predistortion is one of the best ways of the many methods of nonlinear compensation. In this technique, the inverse amplitude and phase nonlinearities are added to the TWTA/SSPA input signal to cancel the TWTA/SSPA output nonlinearities. A predistortion circuit should be capable of generating both inverse amplitudes and phase nonlinearities to achieve a substantial reduction in the IMD products.
Most of the prior art distortion circuits have used either diodes or MESFETs as the nonlinear elements along with associated complex circuits, such as phase shifters, attenuators, etc. One prior art circuit as exemplified by U.S. patent application Ser. No. 608,415, filed May 9, 1984, employs two signal paths, one to change the relative phase angle and the other to introduce a predetermined amount of non-linear distortion. A potential problem with this type of circuit is that as the active and passive circuit elements age, they may change characteristics such that the IMD will increase and degrade the performance of the amplifier.